Add tags to existing images

**What is the problem you're trying to solve**
We would like to be able to add tags to images after they have been uploaded to the ACR. The reason for this is that we upload images, test them and then need to be able to tag them as stable.

**Describe the solution you'd like**
The addition of a command which would allow the user to specify the image manifest/existing tag and then add a tag to the manifest. Maybe as an extension to az acr repository update or as its own command (az acr repository tag).